Electrical harmonic rebalancing apparatus



Aug. 31, 1948. R. F. WILD 2,448,055

ELECTRICAL HARMONIG RBBALANCING APPARATUS Filed April 25, 1944 v 4Sheets-Sheet 1 lOl ' AMPLIFIER INVENTOR. RUDOLF F. WILD ATTORfY.

F. WILD ELECTRICAL HARMONIC REBA-LANCING unnnus Filed April 25. 194

Aug. 31, 1948.

4 Sheets-Sheet 3 FIG FIG.

FREQUENCY FREQUENCY I 0 Bl 3 INVENTOR. RUDOLF F. WILD ATTOR Y.

7 FIG.

FIG. [6

Aug. 31, 1948. I w|| D 2,448,065

ELECTRICAL HARIONIC REBALANCING APPARATUS Filed April 25, 1944 4Sheets-Sheet 4 FIG. I?

FIG. I8

I09 k mow-E? INVENTOR. RUDOLF F WILD ATToR Y.

A Patented Aug. 31, 1948 ELECTRICAL HARMONIC REBALANCING APPARATUSRudolf F. Wild, Philadelphia County, Pa., as-

signor to The Brown Instrument Company, Philadelphia, Pa., a corporationof Pennsylvania Application April 25, 1944, Serial No. 532,692

26 Claims.

The present invention relates to the art of accurately measuring minuteelectrical currents or potentials.

A general object of the invention isto provide an improved method of andapparatus for eliminating the objectionable effects of stray fluctuatingelectrical fields or currents upon the operation of apparatus designedto accurately measure the -magnitude and changes in magnitude of minuteelectrical currents or potentials.

A more specific object of the invention is to provide an improved methodof and apparatus for eliminating the objectionable efiects ofextraneously introduced alternating currents upon the operation ofmeasuring apparatus employed to make accurate measurements of minuteunidirectiohal currents or potentials in low resistance circuits.

A further and more specific object of the invention is to provide animproved measuring and/or controlling instrument of the self-balancingtype which may follow the approved practices of the art in respect tomany of its features, such as adjustment of the exhibiting andrebelancing elements by continuously operable rotatable motor means, andwhich incorporates suitable means to eliminate or at least appreciablyminimize, the disturbing effects of stray electrical fields upon theoperation of the motor means and thereby upon the measuring and/orcontrolling function obtained.

Another object of the invention is to provide an improved method of andapparatus for controlling the energization of a reversible electricalmotor for rotation in one direction or the other accordingly as aregularly fluctuating control voltage is of one phase or of oppositephase and having the desirable characteristic of being unaffected by thepresence oi-extraneous fluctuating currents which may be superimposedupon the control voltage even though the extraneous currents are of thesame fundamental frequency as the control voltage or contain otherfrequency components which have proven troublesome in the prior artarrangements.

A serious problem in the measurement of minute electrical currents orpotentials whether fluctuating or steady in character, and particularlyin the measurement of minute unidirectional current or potentialvariations in low resistance circuits, is the difiiculty of electricallyamplifying such current or potential variations with the high degree offidelity and freedom from extraneous disturbing influences which arerequired for precision measurements. Electronic amplifying arrangementshave been proposed in the prior art which are capable of amplifyingminute alternating or pulsating voltages, but the Y practicalapplication of such amplifiers has been seriously limited by thedisturbing and loading effects of stray fluctuating electrical currentswhich are unavoidably superimposed upon the minute alternating orpulsating current or voltage under measurement. Such extraneous currents cause false balance points of the self-balancing measuringinstruments and also cause erratic and unstable operation thereof, andare especially troublesome when they are of the same frequency as thefundamental frequency of the alternating or pulsating current orpotential under measurement. This is a condition often encountered inpractice since a principal source of stray interfering electricalcurrents which have a disturbing effect upon the operation of themeasuring apparatus is the power mains which supply alternatingelectrical current to the apparatus and from which the alternating orpulsating current or potential under measurement ordinarily is derived.

In the amplification of minute unidirectional electric current orpotential variations in low resistance circuits some'means must beprovided for translating those current or-potential varia-' tions intoalternating or pulsating currents or potentials which are capable ofbeing amplified by the available electronic amplifiers and fordistinguishing between such derived alternating or pulsating current orpotential variations and stray interfering currents or potentials whichmay be superimposed thereon. It is noted that in the present state ofdevelopment of electronic amplifying equipment, conversion of the minuteunidirectional current or potential under measurement into analternating or pulsating current or potential is necessary in order' toaccomplish the amplification of minute unidirectional potential orcurrent variations in low resistance circuits because changes in therelative spacing of the electrodes of electronic amplifying tubes andsmall variations in the amplifier energizing voltages produce currentchanges in the output circuits of the amplifiers which are similar toand are of the same order of magnitude as the unidirectional potentialor current variations under measurement, thus precluding the directamplification of such variations.

The present invention wasspecifically devised for the purpose ofproviding an improved method of and apparatus for overcoming theaforementioned difllculties and relies for its operation upon acharacteristic of the wave form of the energizing current supplied themeasuring apparatus from the power mains, namely that the fundamentalfrequency component is by far predominant although some harmonicfrequency components may also be present. Thatis to say, the amplitudesof the harmonic frequency components are relatively small in comparisonto the amplitude of the fundamental frequency component. For example,the third harmonic frequency component of the ordinary 60 cyclecommercial current source is rarely, if ever, greater than 5% of thefundamental 60 cycle component. Advantage is taken of thischaracteristic in the measurement of minute unidirectional potentials orcurrents, for example, by providing a suitable conversion arrangement toderive from the unidirectional potentials or currents a square wavealternating voltage having a third harmonic of appreciable amplitude,When the square wave alternating voltage so derived is a perfect squarewave, the third harmonic frequency component will be of appreciablemagnitude, approaching a value which is approximately 40% of the am--plitude of the fundamental frequency component of the square Wave. Thissquare wave voltage is impressed 0n the input terminals of an electronicamplifier and the output terminals of the amplifier are connected to theinput terminals of a motor drive stage arranged to eflect selectiveenergization of a reversible electrical motor for rotation in onedirection or the other accordingly as the third harmonic frequencycomponent of said square wave voltage is or one phase or of oppositephase. To this end a. suitabl filter is operatively connected to theelectronic amplifier to attenuate all of the frequency components of thesquare wave alternating voltage being amplified except the thirdharmonic frequency component and to permit only the latter to be appliedto the motor drive' stage for selectively controlling the direction ofmotor rot tation. The filter may desirably comprise a frequencyselective feedback network connected between the output and inputterminals of the amplifier whereby such attenuation may be accomplishedefficiently, and in addition, the loading effect of the attenuatedcomponents upon the amplifier may be materially reduced.

Any stray electrical currents or potentials which may be superimposedupon the controlling square wave alternating voltage will also bimpressed on the input terminals of the electronic amplifier, but due tothe action of the filter, all of the stray frequency components will beattenuated and excluded from the input circuit of the motor drive stageexcept those corresponding to the third harmonic frequency component ofthe controlling voltage. When the stray electrical current or potentialhas no component of the same frequency as that of the third harmonic ofthe controlling voltage, the objectionable effects of the stray currentsor potentials upon the operation of the reversible motor will beentirely eliminated.

Stray potentials or currents having the same frequency as the thirdharmonic frequency com= ponent of the controlling voltage will, ofcourse, pass through the filter and tend to adversely affect theenergization of the reversible motor for rotation. The principal sourceof such a stray frequency component, as has previously been noted, isthe third harmonic frequency component of the energizing currentsupplied to the amplifier and motor from the power mains, which thirdharmonic component, is rarely, if ever, larger than 5% of thefundamental stray component. On the other hand, the third harmoniccomponent of the controlling voltage has an amplitude of approximately40% that of the fundamental component of the controlling voltage.Inasmuch as the amplitude of the controlling current or voltagecomponent passed through the filter has been reduced only to 40%, whilethe stray current or voltage component has en reduced to a maximum of5%, an increase in signal-to-noise" ratio of at least eight times isobtained by means of the present invention, even under the most adverseoperating conditions.

It will be apparent to those skilled in the art that my invention is notlimited in its application to incorporation in apparatus for makingprecise measurements of unidirectional potential or current variationsbut may also be employed to advantage in making measurements of minutealternating or pulsating currents or voltages having a third harmonicfrequency component of relatively large amplitude, and furthermore, may

also be advantageously employed in remote control or positioning systemswherein it is desired to selectively energize a reversible electricalmotor for rotation in one directionor the other in response to reversalin phase of a small alternat ing or pulsating current or voltage havinga third harmonic frequency component of appreciable amplitude.

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed .to and forming apart of this specification. For a better understanding of the invention,however, its advantages and specific objects obtained with its use,reference should be had to the accompanying drawings and descriptivematter in which is illustrated and described a preferred embodiment ofthe invention.

Of the drawings:

Fig. l is a diagrammatic representation of the use of my invention in aself-balancing potentiometric recording system;

Figs. 2 and 3 illustrate in detail one form of converter which may beemployed in the arrangement of Fig. l;

Figs. 4 through 10 show a number of curves illustrating the operation ofthe arrangement of Fig.

Fig. 11 is a curve illustrating the response of the frequency selectivenetwork of Fig. 1;

Fig. 12 is a curve similar to that of Fig, 11 iilustrating the combinedresponse of the electronic amplifier and frequency selective network ofFig. 1;

Fig. 13 illustrates a modification of the arrangement of Fig. 1;

Figs. l4, l5 and 16 show curves illustrating the operation of themodification of Fig. 13; and

Figs. 1'7 and 18 illustrate further modifications of the arrangement ofFig. 1.

In Fig, 1 there is illustrated in schematic form an arrangementincluding an electronic motor drive system I for producing effects inaccordance with the extent of unbalance of a potentiometric network 2which controls the electronic amplifier and is unbalanced in accordancewith the variations in a minute unidirectional electromotive force to bemeasured, namely, that produced by a thermocouple 3 and in which becauseof the small magnitude of the thermocouple electromotive force it is notdesirable, nor possible,

to have the said effects produced directly by the thermocoupleelectromotive force.

More specifically, an arrangement is illustrated in Fig. l for measuringand visually exhibiting the temperature of a furnace, not shown, towhich the thermocouple 3 is responsive. The thermocouple 3 usually islocated at a position remote from the remainder of the measuringapparatus and has its terminals connected by conductors 3 and 3 to theterminals of the potentiometric measuring network 3 which may be 01 anysuitable type such as the Brown potentiometer'network disclosed in theHarrison et al. Patent 2,150,502 issued on March 14, 1939.

The potentiometric measuring network 3 is of a well-known type, and itis believed to be suflicient for the present purposes to note that thepotentiometric measuring network includes a circuit branch in which thethermocouple 3 is connected and an opposing branch including a source ofknown potential such as a battery 3 and resistors 1 and 3, a variableportion of which may be connected to the opposed branches in accordancewith the adjustment 01' a sliding contact 9 whereby the respectiveeflects of the variable and known sources are made equal and oppositeand the potentiometric measuring network is balanced for a given valueof the electromotive force of the thermocouple 3 with the contact 9 in acorresponding position along the length of resistances I and 3. Theposition of contact 9 thus provides a measure of the value of thethermocouple electromotive force. and may serve as a measure of thetemperature to which the thermocouple is opposed.

Upon change in the temperature to which the thermocouple is subjected,an unbalanced unidirectional potential of one polarity or of theopposite polarity is produced in the potentiometric measuring network 3depending upon the sense of unbalance of the potentiometric measuringnetwork 3, and consequently, upon the direction of thetemperaturechange. The unbalanced direct current potential so producedis impressed on the input circuit of the electronic motor drive system Iwhich, as shown, includes an electronic amplifier II), a vibrator ii, aninput transformer 13, and a motor drive stage l3.

The unidirectional potential applied to the input circuit of the motordrive system I is translated by the vibrator H into a pulsating currenthaving one polarity or the opposite polarity depending upon the sense ofunbalance of the potentiometric network. This pulsating current isconverted into a type of square wave alternating current which isimpressed on and amplified by the transformer i3 and is furtheramplified by the electronic amplifier Ill. The amplified quantityproduced at the output terminals of the amplifier I3 is impressed on theinput terminals of the motor drive stage I3 and operates to control theoperation of the latter as is required to efl'ect selective energizationoi a reversible electrical motor I for rotation in one direction or theother. The motor I is employed to operate a slidewire assembly torebalance the potentiometric network 3 and also serves to adjust anindicating and recording mechanism which is described hereinafter.

The potentiometric measuring network 3 includes three resistances l5, l3and I! connected in one branch. These resistances are formed of materialhaving a substantially zero temperature coefllcient of resistance andare employed for calibration purposes. The battery 3 which engages agear 31.

may conveniently take the form of a dry cell and a dual vernier rheostatcomprising resistances l3 and I3 and electrically connected slidingcontacts 33 and 3| which engage the resistances i3 and I3, respectively,are connected in series in a branch parallel to the branch includingresistances l3, l3 and H. The rheostat may be operated by any suitabletype of knob, not shown, which desirably has a direct mechanicalconnection with the contact 33 and a lost motion connection with thecontact 3|. Upon initial movement of the knob the contact 30 is firstmoved and then contact 3| is moved thereby providing a vernieradjustment. To this end, the resistance I3 is preferably of higher valuethan the resistance i3.

Also connected in parallel with the resistances l3, l3 and I1 is a thirdbranch including two series connected resistances 33 and 33. Theresistance 33 is preferably made of nickel, copper or other materialhaving a positive temperature coefllcient oi resistance and theresistance 33 is made 01' manganin having a substantially zerotemperature coeiiicient of resistance. The resistance 33 in conjunctionwith the resistance 33 operates to compensate for changes in the ambienttemperature to which the cold or reference junction of the thermocouple3 is subjected. The resistance 33 is also provided for standardizationpurposes and has a value such that the potential drop across it is ofthe same magnitude as the potential produced by a standard cell, notshown, but which may be periodically connected to the potentiometricnetwork 3 for standardizingthe latter in the manner disclosed, forexample, in application Serial No. 421,173, filed on December 1, 1941,by Walter P. Wills, which issued as Patent No. 2,423,540 on July 8, 1947.

The slidewire assembly of the potentiometric measuring network 3consists of the resistances I and 3 and the contact 9. Theresistance 1com. prises a coil which is wound around and is insulated from a core34. Cooperating with the slidewire 1 is the resistance 3 whichconstitutes a collector bar and comprises a coil wound around a core 35.The slidewire l and collector bar 3 are electrically connected by thesliding contact 9 which is adjusted along the-length oi the slidewire 1and collector bar 8 by the reversible motor 14 as is required tomaintain the potentiometric measuring network 3 balanced. The terminalsof the slidewire 1 and its core N are connected in parallel to theresistance ii.

The shaft of motor I drives a pinion 36 which Attached to and movablewith the gear 31 is a pulley 33 around which is wound an endless cable33. The cable 39 is connected to the potentiometric rebalancing contact3 so that when the motor it rotates, the contact 3 will be moved in onedirection or the other to rebalance the potentiometric measuringnetwork. One end of the cable 39 runs over a pulley 33 which ispivotally mounted and biased by a spring 3| to take up the slack in thecable. The other end of the cable runs around a stationary pulley 33.

A pen 33 is mounted on the carriage which carries the potentiometerrebalancing contact 3 and is arranged in cooperative relation with arecorder chart 3 4 to thereby provide a continuous record of theadjustments of the rebalancing contact 3 which are required to maintainthe potentiometric measuring network 3 balanced. and accordingly, toprovide a record of the vari- 2,44aoes ations in magnitude of theunknown potential produced by the thermocouple 3. The chart 34- may bea' strip chart as shown and is arranged to be driven in any convenientmanner, as for example, by a unidirectional motor 35 through suitablegearing, not shown, so that a record of thevariations on the unknownpotential will be recorded as a continuous line on the chart 34.

The electronic motor drive system I is connected to and receivesenergizing current from alternating current supply mains 36 and 31through a pair of conductors 38 and 39. A double pole-single throwswitch 46 is provided between the motor drive system I and the supplymains 36 and 31' for disconnecting the motor drive system from thesupply mains when it is so desired. Preferably, the switch 40 is solocated that when it is adjusted to the positionto deenergize the systemI it also deenergizes the chart driving motor 35.

One input terminal of the electronic motor drive system I comprises thepoint of engagement of a pair of primary windings 4i and 42 provided onthe input transformer I2. This input terminal is that to which oneterminal of the thermocouple 3 is connected by the conductor 4. Theprimary windings 4I and 42 of the transformer I2 are wound around a corestructure 43 on which is also wound a secondary winding 44. A shield 45is provided between the primary windings 4i and 42 and the secondarywinding 44. The windings 4!, 42 and 44 and the core structure 43 andshield 45 are all housed in a suitable metallic casing.

The converter II illustrated schematically in Fig. 1 and in greaterdetail in Figs. 2 and 3 operates to convert the unbalancedunidirectional currents derived from the potentiometric measuringnetwork 2 into pulsating currents which alternately fiow through firstthe transformer primary winding 4| and then the primary winding 42 toestablish a type of square wave alternating current flow in thesecondary winding 44 of the transformer I2. It will be understood thatany suitable converter may be employed for this purpose but in order toillustrate an operative embodiment the converter shown in detail inFigs. 2 and 3 may be utilized.

The converter II illustrated in detail in Figs. 2 and 3 is of the typedisclosed and claimed in application Serial No. 421,176 filed onDecember 1, 1941 by Frederick W. Side, which issued as Patent No.2,423,524 on July 8, 1947. The converter i I is provided with a base 46in which are mounted terminals 41, 48, 49, 50, and 52. A plate 53 isscrewed to the base 46 by means of screws 54. A stud 55 provided with ascrew threaded extension 56 is screwed to the lower end of the plate 53by means of a lock washer 51 and a nut 58. The.free end of the stud 56is bifurcated and has spaced apart ends 59 and 60. Located between thespaced ends 59 and 66 are an insulating pad, (not shown), a springcontact arm 6|! carryinga contact 62, a resilient stop 63, an insulatingpad 64, a vibrating reed 65 carrying a contact 66, an insulating pad 61,a resilient stop 68, a spring contact arm 69 carrying a contact Ill andan insulating pad (not shown). These elements are all clamped betweenthe spaced apart ends 59 and 60 by abolt H and a nut 12. The springcontact arms '6! and 69 are provided with ears I3 and 14, respectively,which are electrically connected to the terminals 48 and 49,respectively. The vibrating reed 65 is provided with an ear I5 which iselectrically connected to the terminal 41'. Riveted to the supportingplate 53 are also studs 16 and I! which carry adjustable stops in theform of screws I6 and I9 formed of insulating material. When theadjustable stops [8 and I9 have been adjusted as desired they areclamped in place by means of screws 86 and Ill, respectively. The springcontact arm 69 carrying the contact I6 through its own resiliencyengages the resilient stop 68 and the resilient stop 68 through its ownresiliency engages the adjustable stop 18. In like manner, the sprincontact arm 6| engages the resilient stop 63 which in turn engages theadjustable stop 19. By ad justing the adjustable stops I3 and 19, thepositions of the contacts 16 and 62 may be independently adjusted withrespect to the contact 66 carried by the vibrating reed 65.

A permanent magnet 82 is secured to the supporting plate 53 by screws 83and 64. A coil 65 is heldin place by a bracket 86, which in turn,

. is secured in place by the screws 63 and 84. The

end of the vibrating reed 65 is disposed within the coil and is providedwith an armature 81 which is riveted to the vibrating reed 65 by rivets88 as seen in Fig. 3.

The coil 85 is energized with alternating cur- I rent and acts on thearmature B1 to vibrate the reed 65 at 60 cycles per second when thealterhatin current supplied by the alternating current supply mains 36and 31 is 60 cycle alternating current to cause the contact 66 to engageand disengage the contacts 70 and 62 at the same frequency. Thepermanent magnet 82 operates in conjunction with the coil and thearmature 81' in such manner as to cause the armature 61 to vibrate insynchronism with the alternating current supplied by the mains 36 and31. By adjusting the adjustable stops I8 and I9 and hence the contactsi6 and 62, the wave form produced by the contacts 62, 66, and III may beadjusted to the desired value and shape. The contacts 62 and I6 arepreferably so arranged that when the contact 66 is in its stationaryposition, it engages both contacts IIl'and 62. This provides anoverlapping action which compensates for wear of the contacts and alsocontributes somewhat to the elimination of stray electrical effects onthe operation of the apparatus. Due to this overlapping action also wearof the contacts does not materially alter the wave form produced by thecontacts. By mountin the contacts 62 and 10 on the spring contact arms6i and 69, respectively, good wiping contact is at all times provided bythe contact 66 and the contacts 62 and I0. An electrical connection maydesirably be provided between one of the screws 54 and ground so thatthe various parts of the converter may be connected to ground tomaintain the converter at ground potential. A cover (not shown) may alsodesirably be provided for enclosing the movable parts of the converter.Such a cover may be held in place on the base 46 by means of 2. rolledflange clamping the cover to the base. Such a cover will act to preventdirt and corrosive atmospheres from affecting the parts of theconverter.

The converter II is essentially apolarizing switching mechanism, theoperating winding 85 and the permanent magnet 82 cooperating to vibratethe reed 65 at 60 cycles per second in synchronism with the 60 cyclealternating current supplied by the mains 36 and 31. For purposes ofexplanation, it may be assumed that the contact I6 is engaged by thecontact 66 during a first half cycle of the alternating current supplywhen the voltage is positive and the second contact 62 is engaged by thecontact 99 during the second half cycle when the alternating voltagesupply is negative. Accordingly, the contacts 99 and I9 engage when thevoltage of the alternating current supply is positive and the contacts99 and 92 engage when the voltage of the alternating current supply isnegative. When the vibrating reed 99 is stationary in its mid-positionboth contacts 92 and 19 will be engaged by the contact 99 so thatwhenthe vibrating reed is operated the contact 99 is always inengagement with one or the other of the contact 92 and 19.

As shown in Fig. 1, the contact 92 of the converter II is connected by aconductor 99 to the terminal of the transformer primary winding 4| whichis remote from the primary winding 42. Similarly, the contact 19 isconnected by a conductor 99 to the terminal of the primary winding 42which is remote from the primary winding 4|.

As the vibrating reed 99 vibrates, therefore, the transformer primarywindings 4| and 42 will alternately be connected in a series circuitwhich may be traced from one terminal or the thermocouple 3 throughconductor 4, one or the other of the transformer primary windings 4| and42, the vibrating reed 99 to the potentiometric network point 9|,contact 9 of the potentiometric network slidewire assembly, collectorbar 9 and conductor 9 back to the other terminal of the thermocouple 3.For convenience, the point of engagement of contact 9 and the slidewireresistance 1 has been designated by the reference numeral 92.

With the arrangement described, the flow and direction of current flowthrough the circuit branch from the potentiometric network point 9| tothe converter II, the transformer l2 and the thermocouple 3 to thepotentiometric network point 92 depends upon the relation betweentheelectromotive force produced by the thermocouple 3 and the potentialdiflference between the alternating current supply mains.

ance 1 and the collector bar 8 by the reversible motor l4. The motor i4has a pair of terminals 93 and 94 which are connected in the outputcircuit of the motor drive stage l3 and also has a pair of terminals 95and 96 which are connected to the alternating current supply mains 31and 36 through the switch 40.

The motor I4 comprises a, rotor 91 and two pairs of oppositely disposedfield poles on one pair of which a winding 98 is wound and on the otherpair of which a winding 99 is wound. Winding 99 has its terminalsconnected to the motor terminals 95 and 99 and is supplied withenergizing current from the alternating current supply mains 39 and 31through a condenser Hill of suitable value. Due to the action ofcondenser I99, the current which flows through the motor winding 99 willbe in phase with the voltage of the The winding 99 has its terminalsconnected to the motor terminals 93 and 94 and is supplied withenergizing current from the output circuit of the motor drive stage Hi.The current supplied to the winding 99 from the motor drive stage l3either leads or lags by approximately 90 the voltage of the alternatingcurrent supply mains and establishes a field in the motor rotor 91 whichis displaced 90 in one direction or the other with respect to thatestablished therein by the winding". The reaction between the field setup by the winding 99 with that set up by the winding 99 establishes arotating field in the rotor which rotates in one direction or the otherdepending upon whether the winding 99 is energized with current whichleads or lags by approximately 90 the voltage supplied by the supplymains 36 and potentiometric network points 9| and 92. The

thermocouple 3 is so connected to the potentiometric circuit that theelectromotive force of the thermocouple 3 opposes the potentialdifference between the potentiometric network points 9| and 92. Thepotential difference between. the potentiometric network points 9| and92 is increased and decreased by movement of the sliding contact 9 tothe right and to the left, respectively. With a suitable adjustment ofthe sliding contact 9, the potential difierence between thepotentiometric network points 9| and 92 will be made equal and oppositeto the electromotive force produced by the thermocouple 3 and no currentwill fiow through the above traced circuit including the converter iiand the primary windings 4| and 42 of the transformer I2. On an increasein the thermocouple electromotive force above the potential difierencebetween the potentiometric network points 9| and 92 current will fiow inone direction through the converter II and the primary windings 4| and42 of the transformer l2 and such current fiow may then be eliminated bya suitable adjustment of the sliding contact 9 to the right. When theelectromotive force of the thermocouple 3 becomes less than thepotential difference between the potentiometric network points 9| and 92the current fiow through the converter and the primary windings 4| and42 of transformer I: will be in such a direction as to be eliminated bya suitable adjustment of the sliding contact 9 to the left.

As previously noted, the sliding contact 9 is adjusted along the lengthof the slidewire resist- 31, and consequently, in accordance with thedirection of unbalance of potentiometric network 2, as will becomeapparent as the description proceeds. The direction and duration ofrotation of the motor I4 is controlled in accordance with the directionand extent of unbalance or the potentiometric measuring network 2 sothat on rotation of the motor M, the sliding contact 9 is adjusted inthe proper direction to rebalance the potentiometric measuring network2.

The alternating voltage of square wave form which is produced in thesecondary winding 44 of the transformer l2 upon unbalance of thepotentiometric measuring network 2 is applied to the input terminals |0|and I02 of the electronic amplifier I0. Electronic amplifier l0 may beof any suitable type capable of greatly amplifying small alternatingvoltages and is supplied with energizing electrical current from thehigh voltage secondary winding I03 of a transformer |'94 having a linevoltage primary winding I05 which is connected by the conductors 38 and39 and the switch 49 to the alternating current supply malns' 39 and 31.The transformer i114 also includes a low voltage secondary winding I08and a high voltage secondary winding ill! which is provided with acenter tap. Desirably, rectifying means are included in the amplifierfor providing the necessary unidirectional voltages 'for energizing theelectronic tube anode circuits.

By way of'example, it is noted that the electronic amplifier I may be ofthe type disclosed in the aforementioned patent of Water P. Wills.Preferably, the amplifier is of a type in which aacaocs 11 inputterminals of the electronic motor drive stage i3. The motor drive stagel3 may be exactly like the motor drive stage disclosed in theaforementioned Wills patent and includes an electronic valve III] whichis a twin type triode having a pair of triodes Ill and H2 arranged in asingle envelope. Both of the triodes include anode, control electrode orgrid, cathode and heater :lila

ment elements. Energizing current is supplied the heater filaments ofthe triodes through conductors, not shown, from the low voltagetransformer secondary winding W6.

The input circuits of the triodes iii and H2 are connected in paralleland may be traced from the cathodes of both of the triodes through abiasing resistance H3 to-the output terminal I69 of the amplifier I0,and from the output terminal I08 of the amplifier through a condenserlit to both of the control electrodes or grids of the triodes. Aresistance H5 is provided to connect both of the control electrodes orgrids of the triodes directly to the lower or negative terminal ofresistance H3.

Anode voltage issupplied the triode ill from the transformer secondarywinding am through a circuit which may be traced from the left endterminal of the winding ifi'l to the anode of triode Hi, the anode tocathode resistance of triode ill, the cathode biasing resistance M3, aconductor I I6 to the input terminal at of the motor M, the motorwinding 99 to the input terminal 94 of the motor and to the center tapon the transformer secondary winding Hill. A tuning condenser ill ofsuitable value is connected across the motor input terminals 93 and 9dand thereby in parallel with the motorwinding 9%. Anode voltage issupplied the triode M2 through a circuit which maybe traced from theright end terminal of the transformer secondary winding I01 to the anodeof triode M2, the cathode thereof, resistance I I3 and the parallelconnected motor winding 99 and condenser ill to the center tap on thesecondary winding Nil. Thus, the triodes Ill and H2 are employed tosupply energizing current to the motor control winding 99 of the motorH.

The motor i4 is preferably so constructed that the impedance of thewinding 99, when the condenser H1 is connected in parallel thereto, isof the proper value to match the impedance of the anode circuits of thetriodes ill and H2 when the motor is operating in order to obtain themost efficient operation. Preferably, the motor is so constructed thatit has a high ratio of inductive reactance to resistance, for example,of the order of 6 to 1 or 8 to l at the frequency of the energizingcurrent supplied to it. This provides for maximum power during therunning condition ofthe motor with the least amount of heating and alsoprovides a low impedance path for braking purposes.

The condenser I00 connected in series with the motor winding 98 is soselected with respect to the inductance of the winding 98 as to providea series resonant circuit having a unity power factor. By virtue of theseries resonant circuit, the total impedance of the motor winding 98 issubstantially equal to the resistance of the winding, and since thisresistance is relatively low, a large current flow through the winding98 is made possible. This permits the attainment of maximum power andtorque from the motor Hi. In addition, the current flow through themotor winding 98 is in phase with the voltage of the alternating currentsupply conductors 3i and 31 because of the series resonant circuit. Thevoltage across the motor winding 98, however, leads the supply linevoltage by substantially 90 because of the inductance of the winding 98.

The condenser ill which is connected in parallel with the motor winding99 is so chosen as to provide a parallel resonant circuit having a unitypower factor. This parallel resonant circuit presents a relatively highexternal impedance and a relatively low local circuit imped ance. Theexternal impedance is approximately the same as the impedance of theanode circuits of the triodes Hi and H2 and, accordingly, providesefiicient operation. The internal circuit impedance approximates theactual resistance of the winding as, and since this resistance isrelatively low, the impedance of the local circuit is also relativelylow.

For the first half cycle of the alternating voltage produced across theterminals of the transformer secondary winding N1, the anode of thetriode HE is rendered positive with respect to the potential of thecenter tap on the winding Mil. During the second half cycle of thatalternating voltage, the anode of the triode H2 is rendered positivewith respect to the said center tap. Accordingly, the triodes iii and H2are arranged to conduct on alternate half cycles of the alternatingcurrent supplied by the power mains 36 and 3?.

When no signal or grid bias is impressed upon the control electrodes ofthe triodes ill and H2, pulsating unidirectional current of twice thefrequency of the alternating voltage supplied by the power mains 35 andill is impressed on the motor winding dd. When thus energized the motorMl is not urged to rotation in either direction but remains stationary.Due to the relatively high direct current component of the current thenflowing through the motor winding 99, an appreciable damping effect onthe rotor 91 is produced which tends to prevent rotation of the rotor.As a result, if the rotor 91 has been rotating, the damping actionreferred to quickly brings it to a stop. The condenser H1 in shunt tothe motor winding 99 is so chosen that the condenser and motor windingthen provide a resonant cir-' cuit.

When an alternating voltage in phase with or displace-d 180 in phasefrom the voltage of the power mains 36 and 31 and having the samefundamental frequency is impressed on the control electrodes Hi and M2,the magnitude of the pulses of current flowing in the anode circuit ofone triode Hi or H2 will be increased while the magnitude of the pulsesof current flowing in the anode circuit of the other triode will bedecreased. Accordingly, the pulses of unidirectional current supplied tothe motor winding 99 during one half cycle will predom: inate over thosesupplied to the motor winding during the other half cycle. Which anodecurrent will be increased depends upon whether the alternating voltageimpressed on the control electrodes of the triodes II I and H2 is inphase with or out of phase with the voltage of the power mains 38 and31.

Such e'nergization of the motor winding 99 operates to introduce thereinan alternating component of current of the same frequency as thatsupplied bythe power mains 36 and 31.

g This alternating component of current will either 13 current increasedby the prevailing alternating voltage impressed upon the input circuitsof triodes III and H2, and with either phase relation, the two currentsproduce a magnetic field in the motor core structure which rotates inone direction or the other depending upon said phase relation andactuates the rotor 91 for rotation in a corresponding direction.Moreover, when the motor winding 99 is so energized, the direct currentcomponent of current flowing in that winding is decreased with theresult that the rotor damping effect is reduced.

In accordance with the present invention, the selective energization ofthe reversible electrical motor for rotation in one direction or theother is controlled by the third harmonic frequency component of thefundamental frequency of the alternating voltage supplied by the powermains 36 and 31 instead of by thefundamental frequency component. Themotor energization is so controlled in order to eliminate thepossibility of false balance points of the potentiometric measuringnetwork 2, and also to eliminate erratic and unstable operation of theapparatus due to the introduction of interfering alternating currentsinto the potentiometric measuring circuit 2'or into the input circuit ofthe electronic amplifier I from stray alternating fields or otherextraneous alternating current sources which may be present in thevicinity of the measuring apparatus. A particularly troublesome sourceof such stray interfering currents is the power mains 36 and 31 whichsupply frequency components having the same fundamental and harmonicfrequencies as the controlling alternating voltage derived in thetransformer secondary winding 44 from the unbalanced currents of thepotentiometric measuring network 2. In order to eliminate or minimizethe objectionable effects of such stray interfering currents, a filternetwork H8 is provided for attenuating all of the frequency componentsof the alternating voltage being amplified except the third harmonicfrequency component. Hence, only the third harmonic frequency componentsappear at the output terminals I08 and I09 of the amplifier I0 and areimpressed on the input circuits of the triodes HI and H2 of the motordrive stage. A detailed description of the filter network H8 and itsoperation is given hereinafter.

The manner in which the motor drive stage I3 responds to reversal inphase of the thirdharmonic frequency components which appear attheoutput terminals I08 and I09 of electronic amplifier I0 to selectivelyenergize the reversible electrical motor I4 for rotation in onedirection or the other will now be described by reference to the curvesshown in Figs. 4 through 10. In Fig. 4, the solid curve A represents thepositive half cycles of voltage impressed on the anode circuit of thetriode III from the left hand section of the transformer secondarywinding I01, and the dotted curve B represents the positive half cyclesof the voltage impressed on the anode circuit of the triode I I2 fromthe right hand section of the transformer secondary winding I01. Fig. 5shows the third harmonic frequency component which is impressed upon thecontrol electrodes or grids of the triodes III and H2 in parallel whenthe potentiometric measuring network 2 is unbalanced in one direction.Fig. 8 shows the third harmonic frequency component which is impressedon the control electrodes of the triodes III and H2 when thepotentiometric measuring network 2 is unbalanced in the oppositedirection. Fig. 6 illustrates the anode current fiow through the triodeIII during the first half cycle of the supply voltage when the anode oftriode III is positive and the anode of triode H2 is negative and thethird harmonic frequency component of Fig. 5 is impressed upon thecontrol electrodes of triodes H1 and H2. It will be noted that thiscurrent pulse reaches a maximum when the anode voltage impressed on thetriode I H reaches a maximum value. The anode current 'fiow, moreover,persists only for a fraction of the complete half cycle inasmuch as thethird harmonic frequency component of voltage impressed on the IIIswings the said control electrode negative with respect to theassociated cathode during the first and last portions of the half cycle.Since the third harmonic frequency component tends to drive the controlelectrode of triode -I H in the positive direction during the time whenthe anode voltage on the triode III is at a maximum value, a relativelylarge amplitude pulse of current then tends to fiow in that anodecircuit. It should be noted that the transconductance of the triodevaries with the anode voltage and is a maximum value during that portionof the half cycle when the voltage impressed on the control electrode,as seen in Fig. 5, is a maximum.

During the next half cycle when the anode of triode I I2 becomespositive, the third harmonic frequency component permits the flow ofanode current during the first and lastportions of the cycle when theanode voltage is small, but prevents the fiow of anode current when theanode voltage is at its maximum value. Accordingly, during this halfcycle, two small pulses of anode current. one at the beginning and theother at the end of the half cycle, will be conducted by the triode H2.

With this condition of unbalance, therefore, a series of relativelylarge currentpulses symmetrical about the center of the half cycles A ofthe anode voltage and as shown'in Fig. -6 will tervals in which triode III is non-conductive, a series of smaller current pulses, one at thebeginning and the other at the end of the half cycle B, as seen in Fig.7, will be conducted by the triode H2. The pulses of current conductedby triode H2 are much smaller than the pulses conducted by triode IIIbecause as the voltage on the anode of triode H2 and thetransconductance of this triode assume reasonably large values, thethird harmonic component derived from the control voltage and shown inFig. 5 and impressed on the control electrodes decreases, becomes zero,and cuts off the conduction entirely. Accordingly, the pulses of currentsupplied to the motor winding 99 by the triode III during the half cycleA for the condition of unbalance considered will be considerably greaterthan those supplied to the winding 99 during the second half cycle B.Such energization of motor winding 99 produces a magnetic field in themotor core structure which rotates in one direction and actuates therotor 91 for rotation in a corresponding direction.

When the potentiometric measuring network 2 is unbalanced in theopposite direction, the third harmonic component derived from thecontrol voltage reverses in phase as seen in Fig. 8 and causes a pulseof current to be supplied to the motor winding99 by the triode H2 duringthe second half cycle B which predominates over control electrode oftriode.

15 two smaller pulses of current supplied to the winding 99 from thetriode III during half cycle A, as may be seen by reference to Figs. 9and 10. Such energization of motor winding 99 effects actuation of rotor91 for rotation in the opposite direction.

The filter network II8 which is employed in Fig. 1 for attenuating allof the frequency components appearing at the output and I09 oftheelectronic amplifier I except the third harmonic component of thecontrolling voltage comprises a so-called parallel-T network which ismade up of two T networks connected in parallel. This filter network isprovided with input terminals I I9 and I20 and output terminals I2I andI22, and includes adjustable resistances I23, I24 and I25 andcapacitances I20, I21 and I28. The resistances I24 and I25 and thecondenser I20 form one T network, and the condensers I26 and I21 and theresistance I23 comprise the other T network. The input terminals I I0and I20 are connected to the amplifier output terminals I08 and I09through the condenser H5, and the output terminals HI and I22 areconnected to the amplifier input terminals I01! and I02.

The resistances I22, I24 and I20 are ganged together for operation by asingle control, and may be operated by means of the manipulation of aknob I20 to adjust the frequency or range of frequencies at which theattentuation is maximum.

The response curve of the filter network IIO, which has substantiallyzero transmission at a single frequency, the third harmonic component inthe embodiment of my invention disclosed, is illustrated in Fig. 11.This response characteristic is obtained with a filter network utilizingonly resistances and capacitances, but it will be understood that thesame response characteristic may be obtained by other and differenttypes of networks including combinations of resistance, capacitance, andinductance, or mechanical, piezo-electric or magneto-striction devices.

According to the present invention the filter network II! is so designedand adjusted that the network provides maximum attenuation orsubstantially zero transmission at the frequency of the third harmoniccomponent of the control voltage produced in the-transformer secondarywinding 44 and also of the energizing current supplied by the powermains 36 and 01. In addition, the network is so connected between theoutput terminals I00 and I00 of the amplifier I0 and the input terminalsIM and I02 as to feed energy from the output circuit of the amplifier tothe input circuit thereof in opposition to the voltage impressed on theinput terminals IN and I02. It will be understood that, if desired, asuitable electronic amplifier may be inserted between the outputterminals IN and I22 of the network Ill and the input terminals IM andI02 of the amplifier III to increase the eflectiveness of the networkIIO in cancelling out all the irequency components except the thirdharmonic component. With this arrangement, therefore, the net voltageimpressed on the input circuit of the electronic amplifier I0 isconstituted of the voltage derived from the transformer secondarywinding 41 and the voltage applied from the output terminals HI and I 22of the filter network H0.

When the filter network H0 is so connected to the electronic amplifierI0, the latter efiectively amplifies only a single frequency, namelythat of the third harmonic of the control voltage,

terminals II'Il and all other frequencies are attenuated o eliminated soas to yield a response curve of the character illustrated in Fig. 12.The filter network IIB provides a certain amount of transmission at thesaid other frequencies which the electronic amplifier would normallypass and amplify and the voltage obtained at the output terminals I2!and I22 is fed back to the input terminals MI and I02 of the amplifierIn so that all of the frequency components have a phase difference ofapproximately 180 with respect to the voltage applied to the amplifierinput terminais from the transformer secondary winding 44. As a result,the voltage obtained from the network II! tends to cancel out thevoltages of the same frequency applied to the amplifier input terminals,thereby reducing the gain of the amplifier I0 with respect to thosefrequency components, or if the feedback is suiilciently great makingthe attenuation substantially At the frequency or the frequencies wherethe network II8 provides zero or very low transmission or coupling, thenet effective gain oi! the amplifier is at or near its maximum. Theresult is that the transmission characteristic of the combinationcomprising the amplifier I0 and the filter network I I8 has a generalcharacteristic as shown in Fig. 12.

In Fig. 13 I have illustrated, more or less diagrammatically, amodification of the embodiment of my invention shown in Fig. 1 whichincorporates suitable provisions to reduce the magnitude of the smallerof the pulses conducted by one of the electronic valves of the motordrive stage upon unbalance of the potentiometric network 2 to therebyincrease the torque produced by motor I4, or if desired, to effect areversal of the smaller of the said current pulses to the end that thesesmaller pulses of current may actually be made to aid the larger pulsesof current conducted. by the other valve in energizing the motor I4 forrotation rather than detracting from such energization as in thearrangement of Fig.

. 1. Hence, with this modified embodiment of my invention a considerableincrease in the torque produced by motor I4 may be obtained.

' In the modification of Fig. 13 a tube I30 which may include twoidentical tetrodes I3I and I32 within the same envelope is employed inlieu of the twin triode tube IIO of Fig. 1. The tetrodes I3! and I32, ifdesired, may be contained within separate envelopes. It is noted alsothat the tetrodes I3I and I32 may each comprise any suitable pentodeconnected as a tetrode. Each of the tetrodes I3I and I32, as shown inFig. 13, is provided with anode, screen, control electrode or grid,cathode and heater filament elements.

Energizing current is supplied to the heater filaments of the tetrodesI3I and I32 through conductors. not shown, from the low voltagetransformer secondary winding I06.

The input circuits of the tetrodes I3I and I32 are connected in parallelwith each other, as are the input circuits of the triodes III and H2 ofFig. 1, and may be traced from the cathodes of both of the tetrodesthrough a biasing resistance I33 to the output terminal I09 of theelectronic amplifier I0, and from the output terminal I08 of theamplifier through a condenser I34 to both of the control grids of thetetrodes. A resistance I35 is connected between the said control gridsandthe negative terminal of the biasing resistance I33.

A positive unidirectional potential of suitable magnitude is impressedon both of the screen auaoes 17 grids by a battery I" the positiveterminal of which is directly connected to the screen grids and thenegative terminal 01 which is connected to the lower and negativeterminal oi. the biasing resistance I33. The magnitude of theunidirectional potential impressed on the screen grids by battery I 32is chosen in a manner described hereinafter.

Anode voltage is supplied the tetrodes "I and I32 from the transformersecondary winding I" through parallel circuits which are similar tothose by means of which anode voltage is supplied to include in a commonbranch the motor control winding 33 and the parallel connected tuningcondenser II I. Thus, the anode circuits of the tetrodes I3I and I32 areemployed to supply en-' ergizing current to the motor control winding99. The operation of this modification Of my invention and the manner inwhich the magnitude the screen grid voltage is chosen will now beexplained by reference to Figs. 14, and 16 which show the positive hali'cycles of the anode voltage, the control grid voltage, and the anodecurrent values for the tetrodes IN and I32. In Fig. 14 the solid curve Arepresents the voltage impressed on the anode circuit of tetrode I3Ifrom the left end section of the transformer secondary winding I01, andthe dotted curve B represents the voltage impressed on the anode circuitof tetrode I32 from the right end section of the transformer secondarywinding I01. Fig. 15 illustrates the amplified quantity of the thirdharmonic frequency component of the alternating voltage which isimpressed on the input terminals IIII and I02 of the electronicamplifier II when the potentiometric network 2 is unbalanced in onedirection. Upon unbalance of the potentiometric network 2 in theopposite direction the phase of this third harmonic irequency componentwill be exactly reversed. Fig. 16 shows the anode current flows throughthe tetrodes I3I and I32 when the third harmonic irequency component ofFig. 15 is impressed on the control grids. Specifically, the curve C ofFig. 16 represents the current flow in the anode circuit of tetrode I3I,and the curve D represents the current fiow in the anode circuit of thetetrode I32.

Two modes of operation of the modification of my invention shown in Fig.13 are contemplated. According to the first mode of operation, thecircuit constants are so chosen that the peaks E and F of the curveshown in Fig. 15 will produce as little anode current flow as posthetriodes III and H2 of Fig. l and sible in the output circuit of thetetrode I3I while i the peak G will produce as much anode current fiowas possible in the output circuit or tetrode I32. For the idealcondition when j this mode of operation is employed the peaks E and Fwill produce no current fiow whatever in the anode circuit of tetrodeI3I.

According to the second mode oi operation, the circuit constants are sochosen. that the peaks E and F will produce as much current flow aspossible in the anode circuit of tetrode I3I, but in the oppositedirection from that in which current ordinarily flows in the anodecircuit of an electronic valve, the peak G also producing as muchcurrent fiow as possible, in the usual direction, in the anode circuitof tetrode I32.

The attainment of both modes of operation is accomplished by employingthe tetrodes I3I and I32, the screen voltage of which is adjusted to.

a value slightly higher than the anode voltage v18 value whichcorresponds to the grid voltage peaks E and, F. This value of anodevoltage, as may be seen by reference to Figs. 14 and 15, isapproximately one-half the peak value oi the anode voltage.

In this manner advantage may be taken oi the characteristic of a tetrodethat very small anode current flows for small anode voltages, when theanode voltage is less than the screen voltage, and in addition, itsdirection of flow tor increasing anode voltages from a value somewhatless than the screen voltage to a value somewhat higher than the screenvoltage.

Whether a very small anode current flow in the usual direction isobtained or whether an actual reversal in the direction oi anode currentilow occurs depends upon the characteristics of the tetrodes, and inparticular, upon the velocity with which electrons arrive at the anodeand the secondary emission properties of the latter. Consequently, thehigher the anode voltage on the tetrode I3I is for the grid voltagepeaks E and F, being lower than the screen voltage, however, the smallerthe current flow'in the anode circuit or tetrode III will be and thegreater the chance for a reversal in anode current. Thus. the operationof the motor drive stage according to either of the modes of operationcontemplated may be accomplished by proper choice or the tetrodecharacteristics together with a proper choice 01' the anode and screenvoltages.

As those skilled in the art will recognize by referring to Fig. 16, areversal in the direction of current pulses in the anode circuit oftetrode I3I' at the grid voltage peaks E and F is very desirable becausethey actually aid the current pulses D in the anode circuit or tetrodeI32 in actuating the motor to rotation.

While the operation of invention has been described only in connectionwith the operating conditions encountered when the potentiometricnetwork 2 is unbalanced in the direction to derive a third harmonicfrequency component of the phase shown in Fig. 15 for controlling thecontrol grid potentials of the tetrodes I3I and I32, it will beunderstood that unbalance of the potentiometrlc network in the oppositedirection and the production 01' a third harmonic frequency component ofexactly the opposite phase for controlling the control grid potentialsof the tetrodes III and I32 will cause current pulses of the charactershown by curve D in Fig. 16 to fiow in the anode circuit of tetrode I3Iand current pulses as shown by curve C to flow in the anode circuit oftetrode I 32 when operation is according to the second mentioned modecontemplated. Or the current pulses in the anode circuit of tetrode I32may be maintained at a very small value and not flow in the reversedirection when operation is according to the first mentioned modecontemplated. Regardless of which mode of operation is employed,however, themotor I4 under this reverse condition of unbalance will beenergized for rotation in the opposite direction from that in which itis actuated for the direction of unbalance first considered.

When the potentiometer network 2 is balanced, the voltage impressed onthe control grids of the tetrodes from the output circuit of theamplifier Il will be zero and the tetrodes I3I and I32 will be equallyconductive. Hence, the motor I4 will not be actuated for rotation ineither direction and will remain stationary.

that the anode current will reverse this modification of my I accaoeu Itwill be apparent that the unidirectional potential for energizing thescreen electrodes of the tetrodes I3I and I32 need not be derived from abattery I36, as shown, in Fig. 13, but may, if desired} be derived fromthe alternating current supply conductors 36 and 31 shown in Fig. 1- bymeans of suitable rectifier means. Half wave rectifier means or fullwave rectifier means or known type may be utilized, as desired.

It will be understood that the screen electrodes need not necessarily beenergized by a unidirectional potential, and that. if desired, a sourceof alternating potential pose, for example, in the mannerillustrated inFig. 17 wherein the motor drive stage includes two identical pent/odesI31 and I38, which may be contained in separate envelopes or in the sameenvelope, as shown, and each or which is provided with anode,suppressor, screen, control grid, cathode and heater filament elements.Energizing current is supplied through conductors, not shown, to theheater filaments of the pentodes from the low voltage winding Hi6 of atransformer IMa which corresponds generally to the transformer IM ofFigs. 1 and 13 but differs therefrom in that it includes an additionalcenter tapped secondary winding which is designated by thereferencecharacter I39.

The input circuits of the pentodes 831i and I38 have been shown as beingidentical to those of the tetrodes HI and M2 of Fig. 13, and hence, needno further description.

Anode voltage is supplied the pentodes from the transformer secondarywinding it? through par- .allel circuits similar to those employed toprovide the tetrodes of Fig. 13 with anode voltage and which include themotor control winding in a common branch.

Energizing voltage is supplied to the screen elements of each pentodefrom the opposite halves of the transformer secondary winding we inphase with the voltage impressed on the associated anode. To this endthe center tap on winding I39 is connected by a conductor Mil to thelower end of the cathode biasing resistance i313 and therethrough to thecathodes of the pentodes; which as shown, are'connected together, andits opposite-ends are connected by conductor and I42 to an individualone of the screen elements. In addition, the winding I39 is so wound. onthe transformer lllda that the screen element of each pen'tode is drivenpositive during the same half cycle that the associated anode is drivenpositive.

The improved operation of this modification of my invention, as well asthat of Fig. 13, is obtained by virtue of the fact that the screenelements have a controlling influence over the anode currents. By way ofexample, when the pentodes I31 and I38 .are of the SAC? type andsinusoidal voltage of 100 volts peak is impressed on the screen elementsfrom the transformer secondary. winding I39, the screen voltage at thepeaks E and F of the grid voltage curve shown in Fig. 15 isapproximately 50 volts. With this potential of '50 volts on the screen,the anode current is approximately 8 milliamperes when the potentials ofthe control grids is zero. With the peak value of 100 volts on thescreen elements. the anode current is. approximately milliamperes underthe same condition of control grid voltage. Hence, the useful anodecurrent peaks for producing motor rotation are about two and a halftimes greater than the undesired ones.

In Fig. 18 I have illustrated a modification of may be utilized for thispur-' 20 the arrangement shown in Fig. 17 in which the screen elementsare energized with alternatin potential and which may be employedtoadvantage in further improving the ratio of useful anode currentpulses to the undesired anode current pulses. This improved operation isobtained by negatively biasing the screen elements so that the screenvoltage is very low during the occurrence of the control grid voltagepeaks E and F as seen in Fig. 15.

To this end alternating voltage is supplied the screen elements of thepentodes I31 and I38 from the secondary winding I39a of a transformerIfllb which may be identical to the transformer IMa of Fig. 17 except inregard to the secondary winding I39a which is arranged to supplyapproximately twice the voltage supplied by the winding I39, and anegative unidirectional bias voltage is impressed on the said screenelements by a battery I43 which is inserted in the conductor I40.

With this arrangement, on the assumption that the peak value of thealternating potential impressed on the screen elements is 200 volts andthe negative unidirectional bias voltage impressed on the screenelements by battery I43 is volts, the anode current flow at the controlgrid voltage peaks E and F of Fig. 15, is substantially zero, while theanode current flow at the control grid voltage peak G is approximately20 milliamperes, thus providing a material improvement in the ratio ofthe useful to the undesired current pulses supplied to the motor controlwinding 99.

While in accordance with the provisions of the statutes, I haveillustrated and described the best forms of my invention now known tome, it will be apparent to those skilled in the art that changes may bemade in the form of the apparatus disclosed without departing from thespirit of my invention as set forth in the appended claims, and thatcertain features of my invention may sometimes be used to advantagewithout a corresponding use of other features.

Having now'described my invention, what 1 claim as new and desire tosecure by Letters Patent is: r

l. The method of measuring a direct-current electrical quantity toeliminate the effects of stray fluctuating electrical currentssuperimposed thereon which tend to disturb the measurement comprisingthe steps of translating said direct-current electrical quantity onlyinto a fluctuating current having a pronounced harmonic frequencycomponent which is different from the fundamental frequency component ofsaid stray currents, and applying said derived fluctuating current to ameasuring device which is responsive only to currents of substantiallythe same frequency as said harmonic frequency component.

2. The method of measuring a direct-current electrical quantity toeliminate the effects of stray electrical currents of predeterminedfrequency which tend to disturb the measurement comprising the steps oftranslating said direct-current electrical quantity into a fluctuatingcurrent having a fundamental frequency substantially the same as that ofsaid stray electrical currents and having a pronounced third harmonicfrequency component, and applying said derived fluctuating current to ameasuring device which is responsive only to currents of substantiallythe same frequency as said third harmonic frequency component.

3. The method of measuring an unknown elecrical c urrent including aunidirectional component the magnitude of which it is desired to measureand a fluctuating component of extraneous origin it is desired toeliminate comprising the steps of translating said unidirectionalcomponent only into a fluctuating current having a pronounced harmonicfrequency component which is different from the fundamental frequencycomponent of said fluctuating compo-- nent of extraneous origin, andapplying said derived fluctuating current to a measuring device which isresponsive only to currents of substantially the same frequency as saidharmonic frequency component.

4. The method of measuring an unknown electrical current including aunidirectional component the magnitude of which it isdesired to measureand a fluctuating component of extraneous origin it is desired toeliminate comprising the steps of alternately applying said unknownelectrical current to the opposite halves of the primary winding of atransformer having a secondarywinding to establish a fluctuating currentcomponent in said secondary winding correspondingto said unidirectionalcomponent and having a pronounced harmonic frequency component which isdifferent from the fundamental frequency component of said straycomponent and also to establish in said secondary winding 2. secondfluctuating current component corresponding to said fluctuating currentcomponent of extraneous origin, amplifying both of said fluctuatingcurrent components established in said transformer secondary winding,distinguishing between the amplified fluctuating components of differentfrequency and feeding back all of the frequency components except saidhar-, monic frequency component in degenerative manner to substantiallycancel out all of the frequency components except said harmonicfrequency component, and utilizing the amplified quantity of saidharmonic frequency component to actuate an indicating device.

5. The method of selectively controlling the I direction of rotation ofa reversible electrical motor in accordance with the polarity of a,directcurrent electrical quantity to eliminate the effects of strayfluctuating electrical currents superimposed upon said direct-currentelectrical quantity and which tend to disturb the operation of saidmotor comprising the steps of translating said direct-current electricalquantity into a fluctuating current having a pronounced harmonicfrequency component which is different from the fundamental frequencycomponent of said stray currents and is-of one phase or of'opposltephase depending upon the polarity of said direct-current electricalquantity, and controlling the energization of said motor for rotation inone direction or the other in accordance with the phase of said harmonicfrequency component.

6. The method of selectively controlling the direction of rotation of areversible electrical motor in accordance with the polarity of adirectcurrent electrical quantity to eliminate the eflects of strayfluctuating electrical currents superimposed upon said direct-currentelectrical quantity and which tend to disturb the operation of saidmotor comprising the steps of alternately applying said direct-currentelectrical quantity to the opposite halves of the primary winding of atransformer having a secondary winding to establish a fluctuatingcurrent component in said secondary winding corresponding to saiddirect-current electrical quantity and having a pronounced harmonicfrequency component which is different from the fundamental frequencycomponent of said stray currents and is of one phase or opposite phasedepending upon the polarity of said direct-current electrical quantityand also to establish in said secondary winding a second fluctuatingcurrent component corresponding to said stray currents, amplifying bothof said fluctuating, current components established in said transformersecondary winding. distinguishing between the amplifled fluctuatingcomponents of different frequency and feeding back all of the frequencycomponents except said harmonic frequency component in degenerativemanner to substantially cancel out all of the frequency componentsexcept said harmonic frequency component, and controlling theenergization of said motor for rotation in one direction or the other inaccordance with the phase of said harmonic frequency component.

7. Apparatus for measuring a direct-current electrical quantity andoperative to eliminate the effects of stray fluctuating electricalcurrents superimposed thereon which tend to disturb the measurementincluding means to translate said direct-current electrical quantityonly into a fluctuating current having a pronounced harmonic frequencycomponent which is different from the fundamental frequency component ofsaid stray currents, a measuring device which is responsive only tocurrents of substantially the same frequency as said harmonic frequencycomponent, and means to apply said derived fluctuating current to saidmeasuring device.

8. Apparatus for measuring a direct-current electrical quantity andoperative to eliminate the effects of stray electrical currents ofpredetermined frequency which tend to disturb the measurement includingmeans to translate said directcurrent electrical quantity into afluctuating current having a fundamental frequency substantially thesame as that of said stray electrical currents and having a pronouncedthird harmonic frequency component, a measuring device which isresponsive only to currents of substantially the same frequency as saidthird harmonic frequency component, and means to apply said derivedfluctuating current to said measuring means.

9. Apparatus for selectively controlling the direction of rotation of areversible electrical motor in accordance with the polarity of adirectcurrent electrical quantity and operative to eliminate the effectsof stray fluctuating electrical currents upon the operation of saidmotor including means having an input circuit to which saiddirect-current electrical quantity is applied and having an outputcircuit to translate said directcurrent electrical quantity into afluctuating current in said output circuit having a pronounced harmonicfrequency component which is different from the fundamental frequencycomponent of said stray currents and is of one phase or of oppositephase depending upon the polarity of said direct-curre'nt electricalquantity, and phase responsive means connected in said output circuitand responsive only to currents of substantially the same frequency assaid harmonic frequency component to control the energization of saidmotor.

10. Apparatus for selectively controlling the direction of rotation of areversible electrical motor in accordance with the polarity of adirect-current electrical quantity and operative to elimihate theeffects of stray electrical currents of predetermined frequency upon theoperation of said motor including means having an input circuit to whichsaid direct-current electrical quantity is applied and having an outputcircuit to translate said direct-current electrical quantity into afluctuating current in said output circuit having a fundamentalfrequency substantially the same as that of said stray electricalcurrents and having a pronounced third harmonic fremotor in accordancewith the polarity of a directcurrent electrical quantity and operativeto eliminate the effects of stray electrical currents of predeterminedfrequency upon the operation of said motor including means to translatesaid direct-current electrical quantity into a fluctuating currenthaving a frequency substantially the.- same as that of said strayelectrical currents and having a pronounced harmonic frequency.component which is of one phase or of opposite phase depending upon thepolarity of said direct-current electrical quantity, an alternatingcurrent circuit including a pair. of terminals adapted to be connectedto a source of alternating current, a pair of grid controlled electronicdevices having output circuits connectedin opposite phase relation tosaid alternating current circuit and having a common output circuit towhich said electronic devices are connected in parallel relation, meansto apply said third harmonic frequency component to the control grids ofsaid electronic devices to selectively render one of said devices moreconductive than the other in accordance with the phase of said harmonicfrequency component, and a reversible alternating current motor having awinding connected to said terminals and a winding connected to theoutput circuit of said electronic devices. 12. Apparatus for selectivelycontrolling the direction of rotation of a reversible electrical motorin accordance with the polarity of a direct current electrical quantityand operative to eliminate the effects of stray electrical currents ofpredetermined frequency upon the operation of said motor including meansto translate said direct-current electrical quantity into a fluctuatingcurrent having a frequency substantially thesame as that of said strayelectrical currents and: having apronounced third.harmon-ic frequencycomponent which is of one phase or of opposite phase depending upon thepolarity of said directcurrent electrical quantity, an alternatingcurrent circuit having a pair of terminals adapted to be connected to asource of alternating current having the same fundamental frequency asthat of said stray electrical currents, a pair of grid controlledelectronic deviceshaving output cir: cuits connected in opposite phaserelation to said.- alternating current circuit andhaving a com'- monoutput circuit to which said electronic de vices are connected inparallel relation, means to apply said third harmonic frequency.component to the control grids of said electronic 'devices toselectivelyrender one of said devices more con-- ductive than the other accordinglyas said third harmonic frequency component is of one phase oroftopposite phase, and a reversible alternating current motor having awinding connected to said terminals and a winding connected to theoutput circuit of said electronic devices.

\13. Apparatus for selectively controlling the direction of rotation ofa reversible electrical m0- tor accordingly as a controlling alternatingpotential having a pronounced harmonic frequency component is of onephase or of opposite phase and operative to eliminate the effects ofstray alternating potentials which may be superimposed upon saidcontrolling alternating potential including an alternating currentcircuit having a pair of terminals adapted to be connected to a sourceof alternating current having the same fundamental frequency as saidcontrolling alternating potential, a pair of grid controlled electronicdevices having output circuits connected in opposite phase relation tosaid alternating current circuit and having a common output circuit towhich said electronic devices are connected in parallel relation, meansto attenuate all of the frequency components of said controlling andstray alternating potentials except said harmonic frequency componentand to apply the said harmonic frequency component to the control gridsof said electronic devices to selectively render one of said devicesmore conductive than the other accordingly as said harmonic frequencycomponent is of one phase or of opposite phase, and a reversiblealternating current motor having a winding connected to said terminalsand a winding connected to the output circuit 'of said elec tronicdevices.

14. The combination of claim 13 wherein each of said grid controlledelectronic devices includes anode, screen grid, control grid, andcathode elements, the anode and cathode elements of each device beingconnected in said alternating current circuit in such manner that avoltage of predetermined magnitude is applied to the anode of eachdevice at the instant that the harmonic component applied to the,control grid attains a peak value, and means to apply an energizingvoltage of magnitude slightly greater than said predetermined magnitudeto the screen grid of each device.

15. The combination of claim 13 wherein each of said grid controlledelectronic devicesincludes anode, screen and cathode elements and theanodes are connected in said output circuits, and means to apply analternating potential between said screen and cathode elements which isin phase with the alternating potential applied to the associated anodesfrom said alternating current circuit.

16. The combination of claim 13 wherein each of said gridcontrolled-electronic devices includes anode, screen and cathodeelements and the anodes are connected in said output circuits includingmeans to apply a unidirectional potential on both of said screenelements which is negative with respect to the potential of theassociated cathodes, and means to alsoapply an alternatin'g potentialbetween said screen and cathode elements which is in phase with thealternating potential applied to the associated anodes from mentalfrequency as said controlling alternating potential and which may besuperimposed .upon said controlling alternating potential including analternating current circuit having a pair of terminals adapted to beconnected to a source of alternating current having the same fundamentalfrequency as that of said controlling potential, a pair of gridcontrolled electronic devices having output circuits connected inopposite phase relation to said alternating current circuit and having acommon output circuit to which said electronic devices are connected inparallel relation, means to attenuate substantially all of the frequencycomponents of said controlling and stray alternating potentials exceptthe third harmonic frequency component and to apply the said thirdharmonic frequency component to the control grids 01' said electronicdevices to selectively render one of said devices more conductive thanthe other accordingly as said third harmonic frequency component is ofone phase or of opposite phase, and a reversible alternating currentmotor having a winding connected to said terminals and a windingconnected to the output circuit of said electronic devices.

18. The combination of claim 17 wherein each of said grid controlledelectronic devices includes anode, screen and cathode elements and theanodes are connected in said output circuits, and

means to apply a unidirectional potential on both of said screenelements which is positive with respect to the potential of theassociated cathode elements, the magnitude of said unidirectionalpotential being slightly greater than the potential impressed on theanode of the least conductive electronic device at the instant when thethird harmonic frequency component impressed on the control grids ofsaid electronic devices is a maximum positive value.

19. The'combination of claim 17 wherein each of said grid controlledelectronic devices includes anode, screen and cathode elements and theanodes are connected in said output circuits, and means to apply apotential on both of said screen elements which is positive with respectto the potential oi the associated cathode elements, the magnitude ofsaid potential being so related to the magnitude of the alternatingpotential applied to the associated anodes from said alternating current circuit that a reversal in the current flow in the output circuitof the electronic device rendered least conductive will be produced whenthe third harmonic frequency component impressed on the control grids ofsaid electronic devices is near its maximum value.

20. The combination of claim 17 wherein each of said grid controlledelectronic devices includes anode, screen and cathode elements and theanodes are connected in said output circuits, and means to apply analternating potential between said screen and cathode elements which isin phase with the alternating potential applied to the associated anodesfrom said alternating current circuit, the magnitude of said alternatingpotential being so related to the magnitude of the alternating potentialapplied to the associated anodes from said alternating current circuitthat the electronic device rendered least conductive will be renderedsubstantially non-conductive.

21. The combination of claim 17 wherein each of said grid controlledelectronic devices includes anode, screen and cathode elements and theanodes are connected in said output circuits including means to apply aunidirectional potential on both of said screen elements which isnegative with respect to the potential of the associated cathodes, andmeans to also apply an alternating potential between said screen andcathode elements which is in phase with the alternating potentialapplied to the associated anodes from said alternating current circuit,the peak value of the alternating potential applied to said screenelements being approximately twice that of said unidirectional potentialand said unidirectional potential having such a value that the potentialof the screen element of the least conductive electronic device isapproximately the same as that of its associated cathode element whenthe third harmonic frequency component impressed on the control grids ofsaid electronic devices is at its maximum positive value.

22. Apparatus for selectively controlling the direction of rotation of areversible electrical motor accordingly as a controlling alternatingpotential is 01' one phase or of opposite phase including an alternatingcurrent circuit having a pair 01' terminals adapted to be connected to asource oi. alternating current having a fundamental frequency which isone-thirdthat of said controlling alternating potential, a pair of gridcontrolled electronic devices having output circuits connected inopposite phase relation to said alternating current circuit and having acommon connection to which said electronic devices are connected inparallel relation, means to apply said controlling alternating potentialto the control grids of said electronic devices to render one of saiddevices more conductive than the other according to the phase of saidcontrolling alternating potential, and a reversible alternating currentmotor having a winding connected to said terminals and a windingconnected to the output circuit of said electronic devices.

23. The combination of claim 22 wherein each of said grid controlledelectronic devices includes anode, screen and cathode elements and theanodes are .connected in said output circuits, and means to apply apotential on each of said screen elements which is positive with respectto the potential of the associated cathode elements, the magnitude ofsaid potential being approximately the same as the potential impressedon the anode of the least conductive electronic device at the instantwhen said controlling voltage is at a maximum positive value.

24. The combination of claim 22 wherein each .of said grid controlledelectronic devices includes anode, screen and cathode elements and theanodes are connected in said output circuits, and means to apply apotential on both of said screen elements which is positive with respectto the potential of the associated cathode elements,

the magnitude of said potential being so related to the magnitude ofthealternating potential applied to the associated anodes from saidalternating current circuit that a reversal in the current flow in theoutput circuit of the electronic device rendered least conductive willbe produced when the controlling alternating voltage impressed on thecontrol grids of said electronic devices is near its maximum positivevalue.

25. The combination of .claim 22 wherein each ofsaid grid controlledelectronic devices includes anode, screen and cathode elements and theanodes are connected in said output circuits, and means to apply analternating potential between said screen and cathode elements which isin 26. The combination of claim 22 wherein each of said grid controlledelectronic devices includes anode, screen and cathode elements and theanodes are connected in said output circuits including means to apply-aunidirectional potential on both of said screen elements which isnegative with respect to the potential of the associated cathodes, andmeans to also apply an alternating potential between said screen andcathode elements which is in phase with the alternating potentialapplied to the associated anodes from said alternating current circuit,the peak alue of the alternating potential applied to said screenelements being approximately twice that of said unidirectional potentialand said unidirectional potential having such a value that the potentialof the screen element of the least conductive electronic device isapproximately the same as that of its associated'cathode element whenthe controlling alternating voltage is'at its maximum positive value.

RUDOLF F. WILD.

REFERENCES crrEn The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,053,154 La Pierre Sept. 1, 19362,150,006 Panic: et a1. Mar. 7, 1939 2,173,426 Scott Sept. 19, 19392,355,537 Jones Aug. 8, 1944 2,364,483 Side Dec. 5, 1944 2,372,062Dorsman Mar. 20, 1945 2,376,527

Wills May 22, 1945

